1. Field of the Invention
The present invention relates in general to a superconducting wire transposition method, more specifically, to a superconducting wire transposition method and superconducting transformer whose winding is formed of superconducting wire to enable the formation of transpositions.
2. Description of the Related Art
In general, a transformer is a passive electrical device used to transfer an alternating current (or voltage) that is supplied to one winding to another winding by means of electromagnetic induction. The simplest type of transformer consists of at least two electric circuits and one common magnetic circuit for the electromagnetic induction. The electric circuits are formed with windings, and the magnetic circuit is formed of a steel core and arranged so that the magnetic circuit is in opposition to the electric circuit.
As shown in FIG. 1, the transformer includes a core 10, and a plurality of windings 12, 14, 16, such as, a primary winding 12, a secondary winding 14, and a tertiary winding 16.
And, as shown in FIG. 2, the transformer with the structure shown in FIG. 1 further includes an insulator 18 for facilitating the assembly process of the core 10 and the windings 12, 14, 16. After a winding 20 is formed on the insulator 18, the core 10 and the insulator 18 are assembled.
If it is necessary to use a plurality of conductors and a large current for the winding, a transposition A changing the positions of the parallel conductors takes place in order to make current-sharing of the parallel conductors equal.
In general, a traditional winding formed of copper wires requires many wire strands to minimize the winding loss caused by alternating current. Especially, the transformer consuming several hundreds to several thousands amperes of current employs several tens of wire strands. To make it work, a variety of winding methods, helical winding, cylindrical winding, and successive winding for example, have been tried. These methods have been developed to attain the same purposes, such as, minimizing coil size, and reducing insulation and winding loss.
The following will now explain the transposition for minimizing the winding loss.
In the case of using more than two strands of parallel conductors for the winding, a voltage drop occurs between each wire due to the difference of internal resistance of the wire inside of the winding and external resistance of the wire outside of the winding. In addition, an induced voltage difference occurs between the internal wire and the external wire of the winding because of the difference in the number of opposite flux. As a result thereof, a circulating current flows between the parallel conductors, and this in turn increases in the winding loss.
Therefore, the transposition method is developed to minimize the circulating current. What it does is to exchange the positions of the inner wire 20a and the external wire 20b at a proper place so that the number of opposite flux between the internal and external wires can be equal. Preferably, transpositions occur at least one less than the number of wire strands. For instance, if four strands of parallel conductors are used, transpositions should occur at least three times.
In recent years, a number of studies have been paid to the development of a transformer based on superconductivity. Unfortunately however, the superconducting transformer imposes several problems with the occurrence of the above-described transposition.
More specifically speaking, superconductivity is a phenomenon occurring in certain materials when temperatures, magnetic fields, and current are lower than the critical levels thereof, and is characterized by the complete absence of electrical resistance. To apply a superconductor to the winding of electric power equipment including the superconducting transformer, it is important to keep the critical current level to maintain superconductivity.
Especially, in case of manufacturing a winding with a plurality of parallel-connected superconductors to generate a large current flow therein, current-sharing between parallel conductors should be uniform. If this is not observed, that is, if the current-sharing between parallel conductors is not uniform, a particular conductor out of the superconducting parallel conductors may cause current distribution and thus, the critical current value is exceeded. When this occurs, superconductivity is lost, and severe damages can be done on the superconducting winding.
Accordingly, to prevent the current distribution, it is necessary to have transpositions as seen in the copper coil winding.
However, a different approach has to be made to apply the transposition method to superconductors because superconductors are made out of ceramic materials and easily lose their superconductivity when they are bent or twisted under pressure. Thus, the traditional transposition method used in the copper winding is not equally effective for the superconductors.
There is a need, therefore, to suggest a new superconducting wire transposition method that works best for the superconducting transformer.